creds-util: add a concept of "user-scoped" credentials

So far credentials are a concept for system services only: to encrypt or
decrypt credential you must be privileged, as only then you can access
the TPM and the host key.

Let's break this up a bit: let's add a "user-scoped" credential, that
are specific to users. Internally this works by adding another step to
the acquisition of the symmetric encryption key for the credential: if a
"user-scoped" credential is used we'll generate an symmetric encryption
key K as usual, but then we'll use it to calculate

    K' = HMAC(K, flags || uid || machine-id || username)

and then use the resulting K' as encryption key instead. This basically
includes the (public) user's identity in the encryption key, ensuring
that only if the right user credentials are specified the correct key
can be acquired.

1 files changed, 9 insertions, 2 deletions

diff --git a/src/shared/creds-util.h b/src/shared/creds-util.h
index 9362d4e52c..bd189e8adb 100644
--- a/src/shared/creds-util.h
+++ b/src/shared/creds-util.h
@@ -59,6 +59,7 @@ int get_credential_user_password(const char *username, char **ret_password, bool
 
 typedef enum CredentialFlags {
         CREDENTIAL_ALLOW_NULL = 1 << 0, /* allow decryption of NULL key, even if TPM is around */
+        CREDENTIAL_ANY_SCOPE  = 1 << 1, /* allow decryption of both system and user credentials */
 } CredentialFlags;
 
 /* The four modes we support: keyed only by on-disk key, only by TPM2 HMAC key, and by the combination of
@@ -66,11 +67,16 @@ typedef enum CredentialFlags {
  * authenticity or confidentiality, but is still useful for integrity protection, and makes things simpler
  * for us to handle). */
 #define CRED_AES256_GCM_BY_HOST               SD_ID128_MAKE(5a,1c,6a,86,df,9d,40,96,b1,d5,a6,5e,08,62,f1,9a)
+#define CRED_AES256_GCM_BY_HOST_SCOPED        SD_ID128_MAKE(55,b9,ed,1d,38,59,4d,43,a8,31,9d,2e,bb,33,2a,c6)
 #define CRED_AES256_GCM_BY_TPM2_HMAC          SD_ID128_MAKE(0c,7c,c0,7b,11,76,45,91,9c,4b,0b,ea,08,bc,20,fe)
 #define CRED_AES256_GCM_BY_TPM2_HMAC_WITH_PK  SD_ID128_MAKE(fa,f7,eb,93,41,e3,41,2c,a1,a4,36,f9,5a,29,36,2f)
 #define CRED_AES256_GCM_BY_HOST_AND_TPM2_HMAC SD_ID128_MAKE(93,a8,94,09,48,74,44,90,90,ca,f2,fc,93,ca,b5,53)
+#define CRED_AES256_GCM_BY_HOST_AND_TPM2_HMAC_SCOPED            \
+                                              SD_ID128_MAKE(ef,4a,c1,36,79,a9,48,0e,a7,db,68,89,7f,9f,16,5d)
 #define CRED_AES256_GCM_BY_HOST_AND_TPM2_HMAC_WITH_PK           \
                                               SD_ID128_MAKE(af,49,50,a8,49,13,4e,b1,a7,38,46,30,4f,f3,0c,05)
+#define CRED_AES256_GCM_BY_HOST_AND_TPM2_HMAC_WITH_PK_SCOPED    \
+                                              SD_ID128_MAKE(ad,bc,4c,a3,ef,b6,42,01,ba,88,1b,6f,2e,40,95,ea)
 #define CRED_AES256_GCM_BY_NULL               SD_ID128_MAKE(05,84,69,da,f6,f5,43,24,80,05,49,da,0f,8e,a2,fb)
 
 /* Two special IDs to pick a general automatic mode (i.e. tpm2+host if TPM2 exists, only host otherwise) or
@@ -80,6 +86,7 @@ typedef enum CredentialFlags {
  * with an underscore. */
 #define _CRED_AUTO                            SD_ID128_MAKE(a2,19,cb,07,85,b2,4c,04,b1,6d,18,ca,b9,d2,ee,01)
 #define _CRED_AUTO_INITRD                     SD_ID128_MAKE(02,dc,8e,de,3a,02,43,ab,a9,ec,54,9c,05,e6,a0,71)
+#define _CRED_AUTO_SCOPED                     SD_ID128_MAKE(23,88,96,85,6f,74,48,8a,9c,78,6f,6a,b0,e7,3b,6a)
 
-int encrypt_credential_and_warn(sd_id128_t with_key, const char *name, usec_t timestamp, usec_t not_after, const char *tpm2_device, uint32_t tpm2_hash_pcr_mask, const char *tpm2_pubkey_path, uint32_t tpm2_pubkey_pcr_mask, const struct iovec *input, CredentialFlags flags, struct iovec *ret);
-int decrypt_credential_and_warn(const char *validate_name, usec_t validate_timestamp, const char *tpm2_device, const char *tpm2_signature_path, const struct iovec *input, CredentialFlags flags, struct iovec *ret);
+int encrypt_credential_and_warn(sd_id128_t with_key, const char *name, usec_t timestamp, usec_t not_after, const char *tpm2_device, uint32_t tpm2_hash_pcr_mask, const char *tpm2_pubkey_path, uint32_t tpm2_pubkey_pcr_mask, uid_t uid, const struct iovec *input, CredentialFlags flags, struct iovec *ret);
+int decrypt_credential_and_warn(const char *validate_name, usec_t validate_timestamp, const char *tpm2_device, const char *tpm2_signature_path, uid_t uid, const struct iovec *input, CredentialFlags flags, struct iovec *ret);