ASN1_GENERATE_NCONF(3ossl)

📦

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💅 ASN1_GENERATE_NCONF(3ossl) — the OpenSSL manual, bestie edition

okay so. this is the page for ASN1_generate_nconf and ASN1_generate_v3, straight outta the OpenSSL docs. it's giving "build ASN1 structures out of a little string" and honestly? kind of iconic. let's get into it 🎀

✨ NAME ✨

ASN1_generate_nconf, ASN1_generate_v3 — the ASN1 string generation functions. yes bestie, they generate strings. into ASN1. slay.

🌸 SYNOPSIS 🌸

#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 💖

so these two functions generate the ASN1 encoding of a string, and they hand it back to you inside an ASN1_TYPE structure. cute, right?

str is literally the string you wanna encode. nconf or cnf holds the optional configuration info — that's where any additional strings get read from. nconf will typically come from a config file, whereas cnf is the one you get from an X509V3_CTX structure, which is typically used by the X509 v3 certificate extension functions. and like — cnf or nconf can be set to NULL if you're not using any additional configuration. no config, no problem 🫶

🦋 GENERATION STRING FORMAT 🦋

the actual data that gets encoded is decided by the string str plus the configuration info. the general format of the string is:

[modifier,]type[:value]

translating that from nerd: 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 all explained below 👇

⭐ Supported Types ⭐

the supported types are listed below. Case is not significant in the type names. and unless it says otherwise, only the ASCII format is permissible.

BOOLEAN, BOOL — encodes a boolean type. the value string is mandatory and should be TRUE or FALSE. also acceptable: TRUE, true, Y, y, YES, yes, FALSE, false, N, n, NO and no. so many ways to say yes or no, we love options 💅
NULL — encodes the NULL type. the value string must not be present. that's it, that's the whole vibe. nothing. void. empty. 🕳️
INTEGER, INT — encodes an ASN1 INTEGER type. the value string represents the value of the integer, it can be prefaced by a minus sign, and it's normally interpreted as a decimal value unless the prefix 0x is included. hex girlies rise 🔥
ENUMERATED, ENUM — encodes the ASN1 ENUMERATED type. otherwise it's literally identical to INTEGER. same energy, different label ✨
OBJECT, OID — encodes an ASN1 OBJECT IDENTIFIER. the value string can be a short name, a long name, or numerical format. pick your fighter 👑
UTCTIME, UTC — encodes an ASN1 UTCTime structure. the value should be in the format YYMMDDHHMMSSZ. yes, all of those letters, in that order 🕰️
GENERALIZEDTIME, GENTIME — encodes an ASN1 GeneralizedTime structure. the value should be in the format YYYYMMDDHHMMSSZ. it's the UTC format but with the extra Y's, growing up so fast 😭
OCTETSTRING, OCT — encodes an ASN1 OCTET STRING. value represents the contents of this structure, and the format strings ASCII and HEX can be used to specify the format of value.
BITSTRING, BITSTR — encodes an ASN1 BIT STRING. value represents the contents of this structure, and 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. noted 📝
UNIVERSALSTRING, UNIV, IA5, IA5STRING, UTF8, UTF8String, BMP, BMPSTRING, VISIBLESTRING, VISIBLE, PRINTABLESTRING, PRINTABLE, T61, T61STRING, TELETEXSTRING, GeneralString, NUMERICSTRING, NUMERIC — these encode the corresponding string types. value represents the contents of this structure. the format can be ASCII or UTF8. that's a whole lotta string types, and honestly? they ate 💜
SEQUENCE, SEQ, SET — formats the result as an ASN1 SEQUENCE or SET type. value should be a section name which will contain the contents. the field names in that section are ignored, and the values are in the generated string format. if value is absent then an empty SEQUENCE will be encoded. empty seq, no thoughts, head empty 🫧

🎀 Modifiers 🎀

modifiers affect the following structure. you can use them to add EXPLICIT or IMPLICIT tagging, add wrappers, or change the string format of the final type and value. the supported formats are documented below, bestie:

EXPLICIT, EXP — adds 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. so if you don't say anything, that's what you get, no cap.
IMPLICIT, IMP — same as EXPLICIT except IMPLICIT tagging is used instead. that's the whole difference. we love a minimal edit ✨
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. wrap it up, it's cold outside 🧣
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 UTF8 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, and all other bits are zero. zero thoughts, zero bits 💀

🍓 RETURN VALUES 🍓

ASN1_generate_nconf() and ASN1_generate_v3() return the encoded data as an ASN1_TYPE structure, or NULL if an error occurred. sad girl hours 😭

the error codes that can be obtained by ERR_get_error(3).

🌈 EXAMPLES 🌈

okay these are the best part, let's go 💅

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 — it's the key contained in the file client.pem in all OpenSSL distributions. (note: the field names such as 'coeff' are ignored and are present just for clarity. they're decorative. little name tags. 🏷️)

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

and this example is the corresponding public key in a 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 👀

ERR_get_error(3) — go read that one too, bestie, it's the whole "what went wrong" situation 💜

👑 COPYRIGHT 👑

Licensed under the Apache License 2.0 (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at <https://www.openssl.org/source/license.html>.

and that's the page!! 🎀 version 3.5.6, dated 2026-04-07 — mwah, go generate some ASN1, you've got this 🫶✨

📦 where this page comes from

freebsd 15.1: base 15.1
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash-1017 on 2026-11-04: a language model's rewrite of the original, under its terms