Provide key derivation interface functions and a helper to implement the
PRF+ function from rfc4402.
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
cc: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
cc: Chuck Lever <chuck.lever@xxxxxxxxxx>
cc: Marc Dionne <marc.dionne@xxxxxxxxxxxx>
cc: Eric Dumazet <edumazet@xxxxxxxxxx>
cc: Jakub Kicinski <kuba@xxxxxxxxxx>
cc: Paolo Abeni <pabeni@xxxxxxxxxx>
cc: Simon Horman <horms@xxxxxxxxxx>
cc: linux-afs@xxxxxxxxxxxxxxxxxxx
cc: linux-nfs@xxxxxxxxxxxxxxx
cc: linux-crypto@xxxxxxxxxxxxxxx
cc: netdev@xxxxxxxxxxxxxxx
---
crypto/krb5/Makefile | 1 +
crypto/krb5/internal.h | 10 +++
crypto/krb5/krb5_kdf.c | 145 +++++++++++++++++++++++++++++++++++++++++
include/crypto/krb5.h | 10 +++
4 files changed, 166 insertions(+)
create mode 100644 crypto/krb5/krb5_kdf.c
diff --git a/crypto/krb5/Makefile b/crypto/krb5/Makefile
index c450d0754772..8c2050af8fed 100644
--- a/crypto/krb5/Makefile
+++ b/crypto/krb5/Makefile
@@ -4,6 +4,7 @@
#
krb5-y += \
+ krb5_kdf.o \
krb5_api.o
obj-$(CONFIG_CRYPTO_KRB5) += krb5.o
diff --git a/crypto/krb5/internal.h b/crypto/krb5/internal.h
index b542d24e5fa5..50abda5169c7 100644
--- a/crypto/krb5/internal.h
+++ b/crypto/krb5/internal.h
@@ -120,3 +120,13 @@ struct crypto_aead *krb5_prepare_encryption(const struct krb5_enctype *krb5,
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);
diff --git a/crypto/krb5/krb5_kdf.c b/crypto/krb5/krb5_kdf.c
new file mode 100644
index 000000000000..6699e5469d1b
--- /dev/null
+++ b/crypto/krb5/krb5_kdf.c
@@ -0,0 +1,145 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/* Kerberos key derivation.
+ *
+ * Copyright (C) 2025 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@xxxxxxxxxx)
+ */
+
+#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);
+}
diff --git a/include/crypto/krb5.h b/include/crypto/krb5.h
index 4919e135b2ca..a24d9ee53675 100644
--- a/include/crypto/krb5.h
+++ b/include/crypto/krb5.h
@@ -139,6 +139,16 @@ int crypto_krb5_verify_mic(const struct krb5_enctype *krb5,
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);
+
/*
* krb5enc.c
*/
