2025-07-28 15:45:18 +08:00
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RFC4514: String Representation of Distinguished Names
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=====================================================
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Introduction
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------------
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[RFC4514], obsoletes [RFC2253], which defines the standard string format for
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representing *Distinguished Names* (**DN**s) and *Relative Distinguished Names*
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(**RDN**s) within LDAP, but is used more broadly, e.g., as the string
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representations of issuer and subject names in X.509 certificates.
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Distinguished Names (DNs)
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-------------------------
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A *Distinguished Name* (**DN**) is a sequence of *Relative Distinguished Names*
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(**RDNs**).
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### String representation of DNs
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The string representation of a DN consists of strings representing each of its
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`RDNs` separated by commas (`,`). The [RFC4514] specification lists the RDNs
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**in reverse order** (as a result, the most specific elements, such as
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`CommonName`, are output first, and the most general, such as the
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`CountryName`, last). The expected physical order of `RDNs` within a `DN` is
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to list the most general names first.
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Empty DNs are represented by an **empty** string.
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**Example:** `cn=John Doe,ou=People,dc=example,dc=com`
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### Attributes and Values
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Each RDN is composed of one or more attribute-value pairs. An attribute-value
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pair is represented as `type=value`. The type names are not case-sensitive.
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**Example:** `cn=John Doe`. Here `cn` (CommonName) is the attribute type and
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`John Doe` is the attribute value.
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Relative Distinguished Names (RDNs)
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-----------------------------------
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A Relative Distinguished Name (RDN) identifies an entry uniquely within its
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immediate superior entry. An RDN can consist of a single attribute-value pair
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or a set of multiple attribute-value pairs.
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### Multiple Attribute-Value Pairs in an RDN
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The string representation of multiple attribute-value pairs within a single RDN
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separates these by plus signs (`+`). In most cases each RDN consists of just a
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single attribute-value pair. The order of these pairs within an RDN is not
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significant (the ASN.1 abstract syntax designates them as a **SET** rather than
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as a **SEQUENCE**).
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**Example:** `cn=John Doe+uid=jdoe`
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String Representation Rules
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---------------------------
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RFC4514 specifies detailed rules for the string representation of DNs and RDNs
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to handle special characters and ensure unambiguous parsing.
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### Escaping Special Characters
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Certain characters have special meaning in DN strings and must be escaped if
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they appear in an attribute value. The special characters are:
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* Comma (`,`)
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* Plus (`+`)
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* Double Quote (`"`)
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* Backslash (`\`)
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* Less than (`<`)
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* Greater than (`>`)
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* Semicolon (`;`)
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* Leading hash (`#`)
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* Leading or trailing space
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* Optionally escaped: equals (`=`), non-leading hash(`#`)
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These characters are escaped by preceding them with a backslash (`\`). Other
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characters **may** be escaped by encoding **each octet** of their UTF-8
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encoding as two hexadecimal digits preceded by a backslash.
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**Example:** `cn=Doe\, John` (escaping a comma in the value)
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### Hexadecimal Escaping
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Any character **may** be represented separately encoding each byte of its UTF-8
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encoding with its hexadecimal value preceded by a backslash (`\`). This is
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particularly applicable for non-ASCII characters.
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**Example:** `cn=John\20Doe` (representing a space using its hexadecimal code)
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**Example:** `cn=Doe\2c John` (escaping a comma in the value)
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**Example**: `cn=Виктор \d0\94\d1\83\d1\85\d0\be\d0\b2\d0\bd\d1\8b\d0\b9`
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(escaping each UTF-8 byte of the last name).
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### Leading and Trailing Spaces, or a leading hash mark
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Leading or trailing spaces and any leading hash mark in an attribute value must
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be escaped. Spaces in the middle of a value do not need to be escaped.
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**Example:** `cn=\ John Doe\ ` (escaping leading and trailing spaces)
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### Character Sets
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The string must first be converted to UTF-8, prior to any escaping. In
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particular some strings in X.509 certificates may be encoded in 16-bit Unicode
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(BMP) form, as a first step, these need to be converted to UTF-8.
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Tests should include some examples of non-ASCII, non-UTF8 strings that require
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conversion to UTF-8 as part of encoding, the output should not produce the
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`\U<xxxx>` or `\W<xxxxxxxx>` forms seen in `do_esc_char()`.
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Attribute Type Names
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--------------------
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The core attribute type names "c", "l", "o", "ou", etc., are specified directly in
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[RFC4519] Sections 2 and 4. These names are not case sensitive. We may wish
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to expand the set of recognised type names to include some that are new in
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[RFC4519] or in the IANA [LDAP descriptor registry].
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Only the entries of type "A" (Attribute Type) are potentially relevant. All
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the *mainstream* attribute types are already listed in
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`crypto/objects/objects.txt` and should be already supported:
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2025-09-23 03:51:32 +08:00
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| Attribute Name | OID | Reference |
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2025-07-28 15:45:18 +08:00
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| uid | 0.9.2342.19200300.100.1.1 | [RFC4519] |
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| userId | 0.9.2342.19200300.100.1.1 | [RFC4519] |
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| mail | 0.9.2342.19200300.100.1.3 | [RFC4524] |
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| RFC822Mailbox | 0.9.2342.19200300.100.1.3 | [RFC4524] |
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| DC | 0.9.2342.19200300.100.1.25 | [RFC4519] |
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| domainComponent | 0.9.2342.19200300.100.1.25 | [RFC4519] |
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| email | 1.2.840.113549.1.9.1 | [RFC3280] |
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| emailAddress | 1.2.840.113549.1.9.1 | [RFC3280] |
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| cn | 2.5.4.3 | [RFC4519] |
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| commonName | 2.5.4.3 | [RFC4519] |
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| sn | 2.5.4.4 | [RFC4519] |
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| surname | 2.5.4.4 | [RFC4519] |
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| serialNumber | 2.5.4.5 | [RFC4519] |
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| c | 2.5.4.6 | [RFC4519] |
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| countryName | 2.5.4.6 | [RFC4519] |
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| L | 2.5.4.7 | [RFC4519] |
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| localityName | 2.5.4.7 | [RFC4519] |
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| st | 2.5.4.8 | [RFC4519] |
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| stateOrProvinceName | 2.5.4.8 | [RFC2256] |
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| street | 2.5.4.9 | [RFC4519] |
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| streetAddress | 2.5.4.9 | [RFC2256] |
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| o | 2.5.4.10 | [RFC4519] |
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| organizationName | 2.5.4.10 | [RFC4519] |
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| ou | 2.5.4.11 | [RFC4519] |
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| organizationalUnitName | 2.5.4.11 | [RFC4519] |
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| title | 2.5.4.12 | [RFC4519] |
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| description | 2.5.4.13 | [RFC4519] |
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| businessCategory | 2.5.4.15 | [RFC4519] |
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| postalAddress | 2.5.4.16 | [RFC4519] |
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| postalCode | 2.5.4.17 | [RFC4519] |
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| postOfficeBox | 2.5.4.18 | [RFC4519] |
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| physicalDeliveryOfficeName | 2.5.4.19 | [RFC4519] |
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| telephoneNumber | 2.5.4.20 | [RFC4519] |
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| name | 2.5.4.41 | [RFC4519] |
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| givenName | 2.5.4.42 | [RFC4519] |
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| initials | 2.5.4.43 | [RFC4519] |
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| generationQualifier | 2.5.4.44 | [RFC4519] |
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| pseudonym | 2.5.4.65 | [RFC3280] |
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When an attribute type OID is not one of the known values it is represented by
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its dotted-decimal form, and the attribute value must then be encoded with a
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leading `#` character followed by the hexadecimal encoding of the DER encoded
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value, see section 2.4 of [RFC4514]. This form may also be used when the value
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has no suitable string representation.
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I have not checked whether we implement case-insensitive string comparison for
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any of the attributes for which this is expected in LDAP. In certificates I do
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not expect to find case-variants of RDNs that need to be considered equivalent
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when comparing subject and issuer DNs.
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Parsing of Names
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----------------
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The parsing of X.509 directory names (e.g. the `-subj` option of the x509
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command) is performed by the `parse_name()` function in `apps/lib/apps.c`.
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This currently assumes that the output format is that of the legacy
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`X509_NAME_oneline()` function. That format always starts with a `/`
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character. A single slash by itself represents an **empty** RDN sequence.
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The `parse_name()` function is used in the `ca, cmp, req, storeutl,` and
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`x509` commands.
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If or when we switch to output the [RFC4514] format, we need to also accept
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it on input, therefore, `parse_name()` needs to be updated to treat strings
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starting with a `/` as legacy online forms, and other strings as the RFC4514
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format.
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Parsing of [RFC4514] syntax is covered in Section 3. Currently, our parser
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does not support RDNs with ad hoc dott-decimal OIDs, only known named attribute
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types are supported. We should consider allowing explicit dotted decimal OIDs
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and using `X509_NAME_add_entry_by_OBJ()` to add these.
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Names in the configuration file
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-------------------------------
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In configuration files, we represent directory names as a "section" with one
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"attr = value" line per RDN component. Relevant documentation is in
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`x509v3_config(3)` and `openssl-req(1`). For example:
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subjectAltName = dirName:dir_sect
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[dir_sect]
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C = UK
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O = My Organization
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OU = My Unit
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CN = My Name
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So in the configuration file, we only have to handle the syntax of the
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individual value elements, the DN as a whole is not parsed. The `string_mask`
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affects the encoding of the various strings, and defaults to `utf8only` (other
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values are not recommended).
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Only the `ca` and `req` commands process the string mask, though user
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applications can do the same by calling `ASN1_STRING_set_default_mask_asc()`,
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which is an undocumented and non-thread-safe function. The comments above the
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code say:
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/*-
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* This function sets the default to various "flavours" of configuration.
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* based on an ASCII string. Currently this is:
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* MASK:XXXX : a numerical mask value.
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* default : use Printable, IA5, T61, BMP, and UTF8 string types
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* nombstr : any string type except variable-sized BMPStrings or UTF8Strings
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* pkix : PKIX recommendation in RFC 5280
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* utf8only : this is the default, use UTF8Strings
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*/
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Bottom-line is that for most users the DN components in the configuration file
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are already UTF8-friendly, the only thing to check is whether we support the
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desired set of attribute type names, both in the configuration file and while
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parsing a string representation of a complete DN.
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<!-- Links -->
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[RFC2253]:
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<https://www.rfc-editor.org/rfc/rfc2253.html>
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[RFC2256]:
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<https://www.rfc-editor.org/rfc/rfc2256.html>
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[RFC3280]:
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<https://www.rfc-editor.org/rfc/rfc3280.html>
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[RFC4514]:
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<https://www.rfc-editor.org/rfc/rfc4514.html>
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[RFC4519]:
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<https://www.rfc-editor.org/rfc/rfc4519.html>
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[RFC4524]:
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<https://www.rfc-editor.org/rfc/rfc4524.html>
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[LDAP descriptor registry]:
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<https://www.iana.org/assignments/ldap-parameters/ldap-parameters.xhtml#ldap-parameters-3>
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