diff --git a/include/psa/crypto.h b/include/psa/crypto.h
index 1ec9627eb..51d3716cd 100644
--- a/include/psa/crypto.h
+++ b/include/psa/crypto.h
@@ -1228,6 +1228,9 @@ typedef uint32_t psa_algorithm_t;
  * public key, with a key selection or key derivation algorithm to produce
  * one or more shared keys and other shared cryptographic material.
  *
+ * The input to \p kdf_alg is the shared secret `g^{ab}` in big-endian format.
+ * It is `ceiling(n / 8)` bytes long where `n` is the size of the prime `p`.
+ *
  * \param kdf_alg       A key derivation algorithm (\c PSA_ALG_XXX value such
  *                      that #PSA_ALG_IS_KEY_DERIVATION(\p hash_alg) is true)
  *                      or a key selection algorithm (\c PSA_ALG_XXX value such
@@ -1260,6 +1263,10 @@ typedef uint32_t psa_algorithm_t;
  * public key, with a key selection or key derivation algorithm to produce
  * one or more shared keys and other shared cryptographic material.
  *
+ * The input to \p kdf_alg is the shared secret `d_A Q_B = d_B Q_A` in
+ * big-endian format. It is `ceiling(n / 8)` bytes long where `n` is the
+ * curve size in bits.
+ *
  * \param kdf_alg       A key derivation algorithm (\c PSA_ALG_XXX value such
  *                      that #PSA_ALG_IS_KEY_DERIVATION(\p hash_alg) is true)
  *                      or a selection algorithm (\c PSA_ALG_XXX value such