crypto/krb5: Provide infrastructure and key derivation

Provide key derivation interface functions and a helper to implement the
PRF+ function from rfc4402.

Signed-off-by: David Howells <[e-mail]>
cc: Herbert Xu <[e-mail]>
cc: "David S. Miller" <[e-mail]>
cc: Chuck Lever <[e-mail]>
cc: Marc Dionne <[e-mail]>
cc: Eric Dumazet <[e-mail]>
cc: Jakub Kicinski <[e-mail]>
cc: Paolo Abeni <[e-mail]>
cc: Simon Horman <[e-mail]>
cc: [e-mail]
cc: [e-mail]
cc: [e-mail]
cc: [e-mail]

crypto/krb5/Makefile

···
 #
 
 krb5-y += \
+	krb5_kdf.o \
 	krb5_api.o
 
 obj-$(CONFIG_CRYPTO_KRB5) += krb5.o

crypto/krb5/internal.h

···
 struct crypto_shash *krb5_prepare_checksum(const struct krb5_enctype *krb5,
 					   const struct krb5_buffer *Kc,
 					   gfp_t gfp);
+
+/*
+ * krb5_kdf.c
+ */
+int krb5_derive_Kc(const struct krb5_enctype *krb5, const struct krb5_buffer *TK,
+		   u32 usage, struct krb5_buffer *key, gfp_t gfp);
+int krb5_derive_Ke(const struct krb5_enctype *krb5, const struct krb5_buffer *TK,
+		   u32 usage, struct krb5_buffer *key, gfp_t gfp);
+int krb5_derive_Ki(const struct krb5_enctype *krb5, const struct krb5_buffer *TK,
+		   u32 usage, struct krb5_buffer *key, gfp_t gfp);

crypto/krb5/krb5_kdf.c

···
+// SPDX-License-Identifier: GPL-2.0-or-later
+/* Kerberos key derivation.
+ *
+ * Copyright (C) 2025 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells ([e-mail])
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/export.h>
+#include <linux/slab.h>
+#include <crypto/skcipher.h>
+#include <crypto/hash.h>
+#include "internal.h"
+
+/**
+ * crypto_krb5_calc_PRFplus - Calculate PRF+ [RFC4402]
+ * @krb5: The encryption type to use
+ * @K: The protocol key for the pseudo-random function
+ * @L: The length of the output
+ * @S: The input octet string
+ * @result: Result buffer, sized to krb5->prf_len
+ * @gfp: Allocation restrictions
+ *
+ * Calculate the kerberos pseudo-random function, PRF+() by the following
+ * method:
+ *
+ *      PRF+(K, L, S) = truncate(L, T1 || T2 || .. || Tn)
+ *      Tn = PRF(K, n || S)
+ *      [rfc4402 sec 2]
+ */
+int crypto_krb5_calc_PRFplus(const struct krb5_enctype *krb5,
+			     const struct krb5_buffer *K,
+			     unsigned int L,
+			     const struct krb5_buffer *S,
+			     struct krb5_buffer *result,
+			     gfp_t gfp)
+{
+	struct krb5_buffer T_series, Tn, n_S;
+	void *buffer;
+	int ret, n = 1;
+
+	Tn.len = krb5->prf_len;
+	T_series.len = 0;
+	n_S.len = 4 + S->len;
+
+	buffer = kzalloc(round16(L + Tn.len) + round16(n_S.len), gfp);
+	if (!buffer)
+		return -ENOMEM;
+
+	T_series.data = buffer;
+	n_S.data = buffer + round16(L + Tn.len);
+	memcpy(n_S.data + 4, S->data, S->len);
+
+	while (T_series.len < L) {
+		*(__be32 *)(n_S.data) = htonl(n);
+		Tn.data = T_series.data + Tn.len * (n - 1);
+		ret = krb5->profile->calc_PRF(krb5, K, &n_S, &Tn, gfp);
+		if (ret < 0)
+			goto err;
+		T_series.len += Tn.len;
+		n++;
+	}
+
+	/* Truncate to L */
+	memcpy(result->data, T_series.data, L);
+	ret = 0;
+
+err:
+	kfree_sensitive(buffer);
+	return ret;
+}
+EXPORT_SYMBOL(crypto_krb5_calc_PRFplus);
+
+/**
+ * krb5_derive_Kc - Derive key Kc and install into a hash
+ * @krb5: The encryption type to use
+ * @TK: The base key
+ * @usage: The key usage number
+ * @key: Prepped buffer to store the key into
+ * @gfp: Allocation restrictions
+ *
+ * Derive the Kerberos Kc checksumming key.  The key is stored into the
+ * prepared buffer.
+ */
+int krb5_derive_Kc(const struct krb5_enctype *krb5, const struct krb5_buffer *TK,
+		   u32 usage, struct krb5_buffer *key, gfp_t gfp)
+{
+	u8 buf[5] __aligned(CRYPTO_MINALIGN);
+	struct krb5_buffer usage_constant = { .len = 5, .data = buf };
+
+	*(__be32 *)buf = cpu_to_be32(usage);
+	buf[4] = KEY_USAGE_SEED_CHECKSUM;
+
+	key->len = krb5->Kc_len;
+	return krb5->profile->calc_Kc(krb5, TK, &usage_constant, key, gfp);
+}
+
+/**
+ * krb5_derive_Ke - Derive key Ke and install into an skcipher
+ * @krb5: The encryption type to use
+ * @TK: The base key
+ * @usage: The key usage number
+ * @key: Prepped buffer to store the key into
+ * @gfp: Allocation restrictions
+ *
+ * Derive the Kerberos Ke encryption key.  The key is stored into the prepared
+ * buffer.
+ */
+int krb5_derive_Ke(const struct krb5_enctype *krb5, const struct krb5_buffer *TK,
+		   u32 usage, struct krb5_buffer *key, gfp_t gfp)
+{
+	u8 buf[5] __aligned(CRYPTO_MINALIGN);
+	struct krb5_buffer usage_constant = { .len = 5, .data = buf };
+
+	*(__be32 *)buf = cpu_to_be32(usage);
+	buf[4] = KEY_USAGE_SEED_ENCRYPTION;
+
+	key->len = krb5->Ke_len;
+	return krb5->profile->calc_Ke(krb5, TK, &usage_constant, key, gfp);
+}
+
+/**
+ * krb5_derive_Ki - Derive key Ki and install into a hash
+ * @krb5: The encryption type to use
+ * @TK: The base key
+ * @usage: The key usage number
+ * @key: Prepped buffer to store the key into
+ * @gfp: Allocation restrictions
+ *
+ * Derive the Kerberos Ki integrity checksum key.  The key is stored into the
+ * prepared buffer.
+ */
+int krb5_derive_Ki(const struct krb5_enctype *krb5, const struct krb5_buffer *TK,
+		   u32 usage, struct krb5_buffer *key, gfp_t gfp)
+{
+	u8 buf[5] __aligned(CRYPTO_MINALIGN);
+	struct krb5_buffer usage_constant = { .len = 5, .data = buf };
+
+	*(__be32 *)buf = cpu_to_be32(usage);
+	buf[4] = KEY_USAGE_SEED_INTEGRITY;
+
+	key->len = krb5->Ki_len;
+	return krb5->profile->calc_Ki(krb5, TK, &usage_constant, key, gfp);
+}

include/crypto/krb5.h

···
 			   struct scatterlist *sg, unsigned int nr_sg,
 			   size_t *_offset, size_t *_len);
 
+/*
+ * krb5_kdf.c
+ */
+int crypto_krb5_calc_PRFplus(const struct krb5_enctype *krb5,
+			     const struct krb5_buffer *K,
+			     unsigned int L,
+			     const struct krb5_buffer *S,
+			     struct krb5_buffer *result,
+			     gfp_t gfp);
+
 #endif /* _CRYPTO_KRB5_H */