diff --git a/common/sp800-185.c b/common/sp800-185.c
new file mode 100644
index 00000000..929f8518
--- /dev/null
+++ b/common/sp800-185.c
@@ -0,0 +1,138 @@
+#include <stddef.h>
+#include <stdint.h>
+
+#include "sp800-185.h"
+
+static size_t left_encode(uint8_t *encbuf, size_t value)
+{
+    size_t n, i, v;
+
+    for ( v = value, n = 0; v && (n < sizeof(size_t)); ++n, v >>= 8 )
+        ; /* empty */
+    if (n == 0)
+        n = 1;
+    for ( i = 1; i <= n; ++i )
+    {
+        encbuf[i] = (uint8_t)(value >> (8 * (n-i)));
+    }
+    encbuf[0] = (uint8_t)n;
+    return n + 1;
+}
+
+void cshake128_inc_init(shake128incctx *state, const uint8_t *name, size_t namelen, const uint8_t *cstm, size_t cstmlen) {
+    uint8_t encbuf[sizeof(size_t)+1];
+
+    shake128_inc_init(state);
+
+    shake128_inc_absorb(state, encbuf, left_encode(encbuf, SHAKE128_RATE));
+
+    shake128_inc_absorb(state, encbuf, left_encode(encbuf, namelen * 8));
+    shake128_inc_absorb(state, name, namelen);
+
+    shake128_inc_absorb(state, encbuf, left_encode(encbuf, cstmlen * 8));
+    shake128_inc_absorb(state, cstm, cstmlen);
+
+    if (state->ctx[25] != 0) {
+        state->ctx[25] = SHAKE128_RATE - 1;
+        encbuf[0] = 0;
+        shake128_inc_absorb(state, encbuf, 1);
+    }
+}
+
+void cshake128_inc_absorb(shake128incctx *state, const uint8_t *input, size_t inlen) {
+    shake128_inc_absorb(state, input, inlen);
+}
+
+void cshake128_inc_finalize(shake128incctx *state) {
+    state->ctx[state->ctx[25] >> 3] ^= (uint64_t)0x04 << (8 * (state->ctx[25] & 0x07));
+    state->ctx[(SHAKE128_RATE - 1) >> 3] ^= (uint64_t)128 << (8 * ((SHAKE128_RATE - 1) & 0x07));
+    state->ctx[25] = 0;
+}
+
+void cshake128_inc_squeeze(uint8_t *output, size_t outlen, shake128incctx *state) {
+    shake128_inc_squeeze(output, outlen, state);
+}
+
+void cshake256_inc_init(shake256incctx *state, const uint8_t *name, size_t namelen, const uint8_t *cstm, size_t cstmlen) {
+    uint8_t encbuf[sizeof(size_t)+1];
+
+    shake256_inc_init(state);
+
+    shake256_inc_absorb(state, encbuf, left_encode(encbuf, SHAKE256_RATE));
+
+    shake256_inc_absorb(state, encbuf, left_encode(encbuf, namelen * 8));
+    shake256_inc_absorb(state, name, namelen);
+
+    shake256_inc_absorb(state, encbuf, left_encode(encbuf, cstmlen * 8));
+    shake256_inc_absorb(state, cstm, cstmlen);
+
+    if (state->ctx[25] != 0) {
+        state->ctx[25] = SHAKE256_RATE - 1;
+        encbuf[0] = 0;
+        shake256_inc_absorb(state, encbuf, 1);
+    }
+}
+
+void cshake256_inc_absorb(shake256incctx *state, const uint8_t *input, size_t inlen) {
+    shake256_inc_absorb(state, input, inlen);
+}
+
+void cshake256_inc_finalize(shake256incctx *state) {
+    state->ctx[state->ctx[25] >> 3] ^= (uint64_t)0x04 << (8 * (state->ctx[25] & 0x07));
+    state->ctx[(SHAKE256_RATE - 1) >> 3] ^= (uint64_t)128 << (8 * ((SHAKE256_RATE - 1) & 0x07));
+    state->ctx[25] = 0;
+}
+
+void cshake256_inc_squeeze(uint8_t *output, size_t outlen, shake256incctx *state) {
+    shake256_inc_squeeze(output, outlen, state);
+}
+
+/*************************************************
+ * Name:        cshake128
+ *
+ * Description: cSHAKE128 XOF with non-incremental API
+ *
+ * Arguments:   - uint8_t *output: pointer to output
+ *              - size_t outlen: requested output length in bytes
+ *              - const uint8_t *name: pointer to function-name string
+ *              - size_t namelen: length of function-name string in bytes
+ *              - const uint8_t *cstm: pointer to non-empty customization string
+ *              - size_t cstmlen: length of customization string in bytes
+ *              - const uint8_t *input: pointer to input
+ *              - size_t inlen: length of input in bytes
+ **************************************************/
+void cshake128(uint8_t *output, size_t outlen,
+               const uint8_t *name, size_t namelen,
+               const uint8_t *cstm, size_t cstmlen,
+               const uint8_t *input, size_t inlen) {
+    shake128incctx state;
+    cshake128_inc_init(&state, name, namelen, cstm, cstmlen);
+    cshake128_inc_absorb(&state, input, inlen);
+    cshake128_inc_finalize(&state);
+    cshake128_inc_squeeze(output, outlen, &state);
+}
+
+/*************************************************
+ * Name:        cshake256
+ *
+ * Description: cSHAKE256 XOF with non-incremental API
+ *
+ * Arguments:   - uint8_t *output: pointer to output
+ *              - size_t outlen: requested output length in bytes
+ *              - const uint8_t *name: pointer to function-name string
+ *              - size_t namelen: length of function-name string in bytes
+ *              - const uint8_t *cstm: pointer to non-empty customization string
+ *              - size_t cstmlen: length of customization string in bytes
+ *              - const uint8_t *input: pointer to input
+ *              - size_t inlen: length of input in bytes
+ **************************************************/
+void cshake256(uint8_t *output, size_t outlen,
+               const uint8_t *name, size_t namelen,
+               const uint8_t *cstm, size_t cstmlen,
+               const uint8_t *input, size_t inlen) {
+    shake256incctx state;
+    cshake256_inc_init(&state, name, namelen, cstm, cstmlen);
+    cshake256_inc_absorb(&state, input, inlen);
+    cshake256_inc_finalize(&state);
+    cshake256_inc_squeeze(output, outlen, &state);
+}
diff --git a/common/sp800-185.h b/common/sp800-185.h
new file mode 100644
index 00000000..06781523
--- /dev/null
+++ b/common/sp800-185.h
@@ -0,0 +1,23 @@
+#ifndef SP800_185_H
+#define SP800_185_H
+
+#include <stddef.h>
+#include <stdint.h>
+
+#include "fips202.h"
+
+void cshake128_inc_init(shake128incctx *state, const uint8_t *name, size_t namelen, const uint8_t *cstm, size_t cstmlen);
+void cshake128_inc_absorb(shake128incctx *state, const uint8_t *input, size_t inlen);
+void cshake128_inc_finalize(shake128incctx *state);
+void cshake128_inc_squeeze(uint8_t *output, size_t outlen, shake128incctx *state);
+
+void cshake128(uint8_t *output, size_t outlen, const uint8_t *name, size_t namelen, const uint8_t *cstm, size_t cstmlen, const uint8_t *input, size_t inlen);
+
+void cshake256_inc_init(shake256incctx *state, const uint8_t *name, size_t namelen, const uint8_t *cstm, size_t cstmlen);
+void cshake256_inc_absorb(shake256incctx *state, const uint8_t *input, size_t inlen);
+void cshake256_inc_finalize(shake256incctx *state);
+void cshake256_inc_squeeze(uint8_t *output, size_t outlen, shake256incctx *state);
+
+void cshake256(uint8_t *output, size_t outlen, const uint8_t *name, size_t namelen, const uint8_t* cstm, size_t cstmlen, const uint8_t *input, size_t inlen);
+
+#endif