aes_gcm_decrypt(9)

💅 aes_gcm_decrypt — bestie edition

okay so this is aes_gcm_decrypt(9) from the Kernel API Manual, and it is literally the function that decrypts a message with AES-GCM. she's an icon, she's a legend, and she's about to hand you back your plaintext 🎀

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✨ SYNOPSIS ✨

int aes_gcm_decrypt (u8 *dst , const u8 *src , size_t data_len ,
                     const u8 *authtag , const u8 *ad , size_t ad_len ,
                     const u8 nonce[static 12] ,
                     const struct aes_gcm_key *key );

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🌸 ARGUMENTS 🌸

• dst — where the decrypted plaintext goes, queen. it can be in-place or out-of-place, we love flexibility 💖 but for any other overlap, the behavior is unspecified — so don't get cute with it, bestie, that's an unhinged vibe check you will fail.
• src — the source ciphertext data. the scrambled-up little mystery we're solving 🕵️‍♀️
• data_len — length of the plaintext in bytes (and also the length of the ciphertext excluding the tag). max is at most 2^36 - 32. memorise it. it's giving strict but fair.
• authtag — the stored authentication tag 🏷️ its length is the authtag_len that was passed to aes_gcm_preparekey. usually, protocols using AES-GCM put the tag at the end of the ciphertext — in which case this should be set to src + data_len, and src must have room for the tag. iykyk ✨
• ad — the associated data (yes, the data that travels alongside, no, it doesn't get encrypted, don't ask).
• ad_len — length of associated data in bytes: at most 2^61 - 1. huge. iconic. periodt.
• nonce — the 12-byte nonce. twelve. exactly twelve. not eleven, not thirteen. 💅
• key — the key, already prepared using aes_gcm_preparekey. like, already prepared. do not show up here with a raw key, that's embarrassing for everyone.

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🦋 DESCRIPTION 🦋

For AES-GMAC (i.e., AES-GCM without any data en/decrypted — a vibe-only run, no payload), use dst=NULL, src=NULL, and data_len=0 to verify the AES-GMAC value. yes bestie, that's the whole trick. no data, just trust ✨

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🌈 CONTEXT 🌈

Any context. she does not gatekeep, she does not discriminate, any context is welcome in this house 🫶

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👑 RETURN 👑

• 0 on success 🎉 this is the only case where any decrypted or associated data can be used. like, the only one. if you're not seeing a 0, hands off the data, fr fr.
• -EBADMSG if the message is inauthentic 💀 that's a fake friend, do not trust that message, do not read that message, do not let it into the group chat.

🚨 real talk: anything that isn't 0 means the output is not usable. the whole point of GCM is that the tag check is the gate, and this gate does not accept vibes as authentication.

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🧚 SEE ALSO 🧚

Kernel file ./include/crypto/aes-gcm.h, plus her whole friend group:

• struct aes_gcm_key(9)
• struct aes_gcm_ctx(9)
• aes_gcm_preparekey(9)
• aes_gcm_encrypt(9)
• aes_gcm_init(9)
• aes_gcm_auth_update(9)
• aes_gcm_encrypt_update(9)
• aes_gcm_decrypt_update(9)
• aes_gcm_encrypt_final(9)
• aes_gcm_decrypt_final(9)

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and that's the page, bestie 💜 this queen lives over in ./include/crypto, she's dated October 2026, and she ships as part of the Kernel API Manual — go decrypt something responsibly, mwah 🎀✨