X509_ALGOR_DUP(3)

💅 X509_ALGOR_DUP(3) — like, the whole algorithm-identifier squad

okay so bestie, this little page lives in the Library Functions Manual and honestly? it's giving "the glue that holds every certificate together." it's X509_ALGOR_DUP(3), and it's about algorithm identifiers. buckle up, it's actually kind of iconic 🎀

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✨ NAME ✨

X509_ALGOR_new, X509_ALGOR_free, X509_ALGOR_dup, X509_ALGOR_set0, X509_ALGOR_get0, X509_ALGOR_cmp — create, change, and inspect algorithm identifiers 💫

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🌸 SYNOPSIS 🌸

so you're pulling from the library “libcrypto” and you gotta grab this header first:

#include <openssl/x509.h>

then the whole friend group shows up like this:

X509_ALGOR *
X509_ALGOR_new(void);

void
X509_ALGOR_free(X509_ALGOR *alg);

X509_ALGOR *
X509_ALGOR_dup(X509_ALGOR *alg);

int
X509_ALGOR_set0(X509_ALGOR *alg, ASN1_OBJECT *aobj, int ptype,
    void *pval);

void
X509_ALGOR_get0(const ASN1_OBJECT **paobj, int *pptype,
    const void **ppval, const X509_ALGOR *alg);

int
X509_ALGOR_cmp(const X509_ALGOR *a, const X509_ALGOR *b);

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🦋 DESCRIPTION 🦋

okay so an X509_ALGOR object represents an ASN.1 AlgorithmIdentifier structure, and that's the one defined in RFC 5280 section 4.1.1.2 — yes, the real one, no cap. what it does is specify a cryptographic algorithm using an ASN.1 object identifier (OID) that you can get from OBJ_nid2obj(3), plus optional algorithm-specific parameters of the type ASN1_TYPE — peep ASN1_TYPE_set(3) for that. and like, X509_ALGOR objects are used by sooo many other objects — for example certificates, certificate revocation lists, and certificate requests. main character energy honestly 👑

X509_ALGOR_new() allocates a whole new X509_ALGOR object, and it comes pre-loaded with the object that OBJ_nid2obj(3) returns for NID_undef as the algorithm, plus a NULL pointer as the parameters. blank slate, clean girl aesthetic ✨

X509_ALGOR_free() frees alg and any data contained in it. if alg is NULL, no action occurs — nothing happens, we simply don't ✋

X509_ALGOR_dup() creates a deep copy of alg. like a full clone, not just a link to the same post 🙈 it's implemented by calling ASN1_item_dup(3) with arguments of X509_ALGOR_it and alg, which is equivalent to calling i2d_X509_ALGOR(3) and d2i_X509_ALGOR(3).

X509_ALGOR_set0() sets the algorithm OID of alg to aobj, and the associated parameter type to ptype with value pval. now the details, because this is where it gets juicy:

• If ptype is V_ASN1_UNDEF, the parameter is omitted and pval is ignored.
• If ptype is zero, pval is ignored and the existing parameter is left unchanged — or, if alg does not contain a parameter, a new, empty parameter of type V_ASN1_UNDEF is added.
• Otherwise ptype and pval have the same meaning as the type and value parameters to ASN1_TYPE_set(3).

ownership of aobj and, unless it is ignored, of pval is transferred to alg on success. so like, hand them over, they're hers now 💅

X509_ALGOR_get0() returns alg's algorithm OID in *paobj, its parameter type in *pptype, and its parameter value in *ppval. any of paobj, pptype, and ppval can be NULL. and heads up — if pptype is NULL or if *pptype is V_ASN1_UNDEF, then *ppval's value is undefined ⚠️ so don't go reading it, bestie.

X509_ALGOR_cmp() compares a and b. that's it, that's the vibe check 🔍

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💖 RETURN VALUES 💖

• X509_ALGOR_new() and X509_ALGOR_dup() return a new X509_ALGOR object or NULL if an error occurs.
• X509_ALGOR_set0() returns 1 for success or 0 if alg is NULL or memory allocation fails.
• X509_ALGOR_cmp() returns 0 if a and b have identical encodings or non-zero otherwise.

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🌈 SEE ALSO 🌈

go read these next, they're all linked up like a little friend group 🫶

ASN1_TYPE_set(3), d2i_X509_ALGOR(3), EVP_DigestInit(3), OBJ_nid2obj(3), X509_get0_signature(3), X509_new(3), X509_PUBKEY_get0_param(3), X509_signature_dump(3)

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⭐ STANDARDS ⭐

RFC 5280: Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile — yes, the full name, we respect the classics 📜

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💀 HISTORY 💀

the lore goes deep, ngl:

• X509_ALGOR_new() and X509_ALGOR_free() appeared in SSLeay 0.4 or earlier and have been available since OpenBSD 2.4.
• X509_ALGOR_dup() first appeared in SSLeay 0.9.1 and has been available since OpenBSD 2.6.
• X509_ALGOR_set0() and X509_ALGOR_get0() first appeared in OpenSSL 0.9.8h and have been available since OpenBSD 4.5.
• X509_ALGOR_cmp() first appeared in OpenSSL 0.9.8zd, 1.0.0p, and 1.0.1k and has been available since OpenBSD 4.9.

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and that's the page, bestie!! brought to you by the good people of Debian, dated June 8, 2025, filed under X509_ALGOR_DUP(3) in the Library Functions Manual 💖✨🎀 go forth and identify those algorithms, slay 🦋