ASN1_GENERATE_NCONF(3)

💖 NAME 💖

ASN1_generate_nconf, ASN1_generate_v3 — the ASN.1 generation functions, aka your little string-to-ASN.1 factories. This is the ASN1_GENERATE_NCONF(3) page from the Library Functions Manual, bestie — yes, that’s the manual-name sitch, and we’re already slaying.

✨ SYNOPSIS ✨

library “libcrypto” #include <openssl/asn1.h>

ASN1_TYPE *
ASN1_generate_nconf(const char *str, CONF *nconf);

ASN1_TYPE *
ASN1_generate_v3(const char *str, X509V3_CTX *cnf);

🦋 DESCRIPTION 🦋

These functions generate the ASN.1 encoding of a string in an ASN1_TYPE structure. Literally the whole vibe.

str contains the string to encode. nconf or cnf contains the optional configuration information where additional strings will be read from. nconf will typically come from a config file, whereas cnf is obtained from an X509V3_CTX structure which will typically be used by X509 v3 certificate extension functions. cnf or nconf can be set to NULL if no additional configuration will be used. No cap.

🌈 GENERATION STRING FORMAT 🌈

The actual data encoded is determined by the string str and the configuration information. The general format of the string is:

[modifier,]type[:value]

That is zero or more comma separated modifiers followed by a type followed by an optional colon and a value. The formats of type, value and modifier are explained below. Stay with me, bestie 💅

🌸 Supported types 🌸

The supported types are listed below. Unless otherwise specified, only the ASCII format is permissible.

Heads up: that ASCII-only rule is giving strict, but it’s in the manual for real.

• BOOLEAN, BOOL — This encodes a boolean type. The value string is mandatory and should be TRUE or FALSE. Additionally true, Y, y, YES, yes, false, N, n, NO and no are acceptable.
• NULL — Encode the NULL type. The value string must not be present.
• INTEGER, INT — Encodes an ASN.1 INTEGER type. The value string represents the value of the integer. It can be prefaced by a minus sign and is normally interpreted as a decimal value unless the prefix 0x is included.
• ENUMERATED, ENUM — Encodes the ASN.1 ENUMERATED type. It is otherwise identical to INTEGER.
• OBJECT, OID — Encodes an ASN.1 OBJECT IDENTIFIER. The value string can be a short name, a long name, or numerical format.
• UTCTIME, UTC — Encodes an ASN.1 UTCTime structure. The value should be in the format YYMMDDHHMMSSZ.
• GENERALIZEDTIME, GENTIME — Encodes an ASN.1 GeneralizedTime structure. The value should be in the format YYYYMMDDHHMMSSZ.
• OCTETSTRING, OCT — Encodes an ASN.1 OCTET STRING. value represents the contents of this structure. The format strings ASCII and HEX can be used to specify the format of value.
• BITSTRING, BITSTR — Encodes an ASN.1 BIT STRING. value represents the contents of this structure. The format strings ASCII, HEX, and BITLIST can be used to specify the format of value. If the format is anything other than BITLIST, the number of unused bits is set to zero.
• BMPSTRING, BMP, GeneralString, IA5STRING, IA5, NUMERICSTRING, NUMERIC, PRINTABLESTRING, PRINTABLE, T61STRING, T61, TELETEXSTRING, UNIVERSALSTRING, UNIV, UTF8String, UTF8, VISIBLESTRING, VISIBLE — These encode the corresponding string types. value represents the contents of this structure. The format can be ASCII or UTF8.
• SEQUENCE, SEQ, SET — Formats the result as an ASN.1 SEQUENCE or SET type. value should be a section name which will contain the contents. The field names in the section are ignored and the values are in the generated string format. If value is absent, then an empty SEQUENCE will be encoded.

💅 Modifiers 💅

Modifiers affect the following structure. They can be used to add EXPLICIT or IMPLICIT tagging, add wrappers, or to change the string format of the final type and value. The supported formats are:

• EXPLICIT, EXP — Add an explicit tag to the following structure. This string should be followed by a colon and the tag value to use as a decimal value. By following the number with U, A, P or C, UNIVERSAL, APPLICATION, PRIVATE or CONTEXT SPECIFIC tagging can be used. The default is CONTEXT SPECIFIC.
• IMPLICIT, IMP — This is the same as EXPLICIT except IMPLICIT tagging is used instead.
• OCTWRAP, SEQWRAP, SETWRAP, BITWRAP — The following structure is surrounded by an OCTET STRING, a SEQUENCE, a SET, or a BIT STRING, respectively. For a BIT STRING the number of unused bits is set to zero.
• FORMAT — This specifies the format of the ultimate value. It should be followed by a colon and one of the strings ASCII, UTF8, HEX, or BITLIST. If no format specifier is included, then ASCII is used. If UTF8 is specified, then the value string must be a valid UTF-8 string. For HEX, the output must be a set of hex digits. BITLIST (which is only valid for a BIT STRING) is a comma separated list of the indices of the set bits, all other bits are zero.

🫶 RETURN VALUES 🫶

ASN1_generate_nconf() and ASN1_generate_v3() return the encoded data as an ASN1_TYPE structure or NULL if an error occurred.

The error codes can be obtained by ERR_get_error(3).

🔥 EXAMPLES 🔥

A simple IA5String:

IA5STRING:Hello World

An IA5String explicitly tagged:

EXPLICIT:0,IA5STRING:Hello World

An IA5String explicitly tagged using APPLICATION tagging:

EXPLICIT:0A,IA5STRING:Hello World

A BITSTRING with bits 1 and 5 set and all others zero:

FORMAT:BITLIST,BITSTRING:1,5

A more complex example using a config file to produce a SEQUENCE consisting of a BOOL an OID and a UTF8String:

asn1 = SEQUENCE:seq_section

[seq_section]

field1 = BOOLEAN:TRUE
field2 = OID:commonName
field3 = UTF8:Third field

This example produces an RSAPrivateKey structure. This is the key contained in the file client.pem in all OpenSSL distributions. Note that the field names such as "coeff" are ignored and are present just for clarity.

asn1=SEQUENCE:private_key
[private_key]
version=INTEGER:0

n=INTEGER:0xBB6FE79432CC6EA2D8F970675A5A87BFBE1AFF0BE63E879F2AFFB93644\
D4D2C6D000430DEC66ABF47829E74B8C5108623A1C0EE8BE217B3AD8D36D5EB4FCA1D9

e=INTEGER:0x010001

d=INTEGER:0x6F05EAD2F27FFAEC84BEC360C4B928FD5F3A9865D0FCAAD291E2A52F4A\
F810DC6373278C006A0ABBA27DC8C63BF97F7E666E27C5284D7D3B1FFFE16B7A87B51D

p=INTEGER:0xF3929B9435608F8A22C208D86795271D54EBDFB09DDEF539AB083DA912\
D4BD57

q=INTEGER:0xC50016F89DFF2561347ED1186A46E150E28BF2D0F539A1594BBD7FE467\
46EC4F

exp1=INTEGER:0x9E7D4326C924AFC1DEA40B45650134966D6F9DFA3A7F9D698CD4ABEA\
9C0A39B9

exp2=INTEGER:0xBA84003BB95355AFB7C50DF140C60513D0BA51D637272E355E397779\
E7B2458F

coeff=INTEGER:0x30B9E4F2AFA5AC679F920FC83F1F2DF1BAF1779CF989447FABC2F5\
628657053A

This example is the corresponding public key in an ASN.1 SubjectPublicKeyInfo structure:

# Start with a SEQUENCE
asn1=SEQUENCE:pubkeyinfo

# pubkeyinfo contains an algorithm identifier and the public key
# wrapped in a BIT STRING
[pubkeyinfo]
algorithm=SEQUENCE:rsa_alg
pubkey=BITWRAP,SEQUENCE:rsapubkey

# algorithm ID for RSA is just an OID and a NULL
[rsa_alg]
algorithm=OID:rsaEncryption
parameter=NULL

# Actual public key: modulus and exponent
[rsapubkey]
n=INTEGER:0xBB6FE79432CC6EA2D8F970675A5A87BFBE1AFF0BE63E879F2AFFB93644\
D4D2C6D000430DEC66ABF47829E74B8C5108623A1C0EE8BE217B3AD8D36D5EB4FCA1D9

e=INTEGER:0x010001

👑 SEE ALSO 👑

ASN1_TYPE_get(3), d2i_ASN1_TYPE(3), x509v3.cnf(5)

💀 HISTORY 💀

ASN1_generate_nconf() and ASN1_generate_v3() first appeared in OpenSSL 0.9.8 and have been available since OpenBSD 4.5.

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xoxo, Debian June 8, 2025 💅 — that’s the footer, bestie: source/version and date, and we are so done with ASN1_GENERATE_NCONF(3). Periodt.