Basic API design (initial commit)

.editorconfig

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+[*.{c,h}]
+indent_style = tab
+indent_size = 8

.gitignore

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+# Prerequisites
+*.d
+
+# Object files
+*.o
+*.ko
+*.obj
+*.elf
+
+# Linker output
+*.ilk
+*.map
+*.exp
+
+# Precompiled Headers
+*.gch
+*.pch
+
+# Libraries
+*.lib
+*.a
+*.la
+*.lo
+
+# Shared objects (inc. Windows DLLs)
+*.dll
+*.so
+*.so.*
+*.dylib
+
+# Executables
+*.exe
+*.out
+*.app
+*.i*86
+*.x86_64
+*.hex
+
+# Debug files
+*.dSYM/
+*.su
+*.idb
+*.pdb
+
+# Kernel Module Compile Results
+*.mod*
+*.cmd
+.tmp_versions/
+modules.order
+Module.symvers
+Mkfile.old
+dkms.conf

LICENSE

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+The MIT License
+
+Copyright (c) 2017 Daan Sprenkels <hello@dsprenkels.com>
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+THE SOFTWARE.

hazmat.h

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+/*
+ * Low level API for Daan Sprenkels' Shamir secret sharing library
+ * Copyright (c) 2017 Daan Sprenkels <hello@dsprenkels.com>
+ *
+ * Usage of this API is hazardous and is only reserved for beings with a
+ * good understanding of the Shamir secret sharing scheme and who know how
+ * crypto code is implemented. If you are unsure about this, use the
+ * intermediate level API. You have been warned!
+ */
+
+
+#ifndef SSS_HAZMAT_H_
+#define SSS_HAZMAT_H_
+
+#include <inttypes.h>
+
+typedef struct {
+	const uint8_t x;
+	const uint8_t y[32];
+} SSS_Keyshare;
+
+
+void SSS_create_keyshares(SSS_Keyshare *out,
+                         const uint8_t key[32],
+                         uint8_t n,
+                         uint8_t k);
+
+
+void SSS_combine_keyshares(uint8_t key[32],
+                          const SSS_Keyshare *shares,
+			  uint8_t k);
+
+
+#endif /* SSS_HAZMAT_H_ */

sss.h

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+/*
+ * Intermediate level API for Daan Sprenkels' Shamir secret sharing library
+ * Copyright (c) 2017 Daan Sprenkels <hello@dsprenkels.com>
+ */
+
+
+#ifndef SSS_SSS_H_
+#define SSS_SSS_H_
+
+#include "hazmat.h"
+#include <inttypes.h>
+
+
+#ifndef SSS_MSGLEN
+/*
+Length of the message (must be known at compile-time)
+*/
+#define SSS_MSGLEN sizeof(uint8_t[64])
+#endif
+
+
+/*
+ * Length of the message authentication code
+ */
+#define SSS_MACLEN sizeof(uint8_t[16])
+
+
+/*
+ * One share of a secret which is shared using Shamir's
+ * the `SSS_CreateShares` function.
+ */
+typedef struct {
+	SSS_Keyshare keyshare;
+	uint8_t ciphertext[SSS_MSGLEN];
+	uint8_t mac[SSS_MACLEN];
+} SSS_Share;
+
+
+/*
+ * Create `n` shares of the secret data `data`. Share such that `k` or more
+ * shares will be able to restore the secret.
+ *
+ * This function will put the resulting shares in the array pointed to by
+ * `out`. The caller has to guarantee that this array will fit at least `n`
+ * instances of `SSS_Share`.
+ */
+void SSS_create_shares(SSS_Share *out,
+                      const uint8_t *data,
+                      uint8_t n,
+                      uint8_t k);
+
+
+/*
+ * Combine the `k` shares pointed to by `shares` and put the resulting secret
+ * data in `data`. The caller has to ensure that the `data` array will fit
+ * at least `SSS_MSGLEN` (default: 64) bytes.
+ *
+ * On success, this function will return 0. If combining the secret fails,
+ * this function will return a nonzero return code. On failure, the value
+ * in `data` may have been altered, but must still be considered secret.
+ */
+int SSS_combine_shares(uint8_t *data,
+                      const SSS_Share *shares,
+	              uint8_t k);
+
+
+#endif /* SSS_SSS_H_ */