KCAPI_CIPHER_DEC_SM4(3)

💖 kcapi_cipher_dec_sm4(3) 💖

okay bestie, so this is a page from the libkcapi Manual, and it's literally about the convenience function kcapi_cipher_dec_sm4_ctr — the SM4 CTR decryption one. yes it's as iconic as it sounds, let's get into it 🎀

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✨ NAME ✨

kcapi_cipher_dec_sm4_ctr — the convenience function for SM4 CTR decryption. it does exactly what it says on the tin, no cap 💅

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🌸 SYNOPSIS 🌸

ssize_t kcapi_cipher_dec_sm4_ctr(const uint8_t * key, uint32_t keylen,
                                 const uint8_t * in, size_t inlen,
                                 const uint8_t * ctr, uint8_t * out,
                                 size_t outlen);

like, seven arguments. she's a lot, but she's worth it 🫶

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🦋 ARGUMENTS 🦋

• key — [in] the key buffer, bestie. the actual key.
• keylen — [in] how long that key buffer is.
• in — [in] the ciphertext data buffer. this is what you're decrypting, mwah.
• inlen — [in] length of the in buffer.
• ctr — [in] the start counter value that gets used for the cipher operation.
• out — [out] the plaintext data buffer. this is where the magic lands ✨
• outlen — [in] length of the out buffer.

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💜 DESCRIPTION 💜

so this convenience function performs an SM4 counter mode encryption operation — using the key you handed it, the input buffer you gave it, and the IV you gave it. the output gets stored in the out buffer. that's the whole vibe.

• The input buffer can be of arbitrary length. no constraints, she's flexible 🌈
• The output buffer must be at least as large as the input buffer. periodt. don't try to be cute and make it smaller.
• The start counter can contain all zeros — and listen, that is not a NULL buffer, it's a buffer full of zeros, two very different things bestie — and it must be exactly 16 bytes in size.
• The SM4 key is fixed 16 bytes long. 16. not 15, not 17. sixteen 💅

then, the return: it gives you back the number of bytes generated upon success; and if something went wrong, you get a negative errno-style error code. so if the number is negative, that's not a byte count, that's a problem, okay? 😭

ngl the zero-vs-NULL counter thing is the classic gotcha here. all zeros = fine, NULL = absolutely not. 🙈

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👑 AUTHOR 👑

• Stephan Mueller <smueller@chronox.de> — Author.

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🌟 COPYRIGHT 🌟

and that's the whole page, bestie! this was libkcapi Manual 1.5.0, dated July 2025, and you were reading KCAPI_CIPHER_DEC_SM4(3) the entire time 💖✨ go decrypt something cute, ily 🫶🎀