diff --git a/knowledge/catalog.json b/knowledge/catalog.json
index 496c3eb..2929620 100644
--- a/knowledge/catalog.json
+++ b/knowledge/catalog.json
@@ -1740,7 +1740,9 @@
       "implementation": "src/hashes/sha1/mod.rs",
       "documentation": "src/hashes/sha1/README.md",
       "tests": [
-        "hashes::sha1::tests::hashes_standard_vectors"
+        "hashes::sha1::tests::hashes_standard_vectors",
+        "hashes::sha1::tests::continues_from_midstate",
+        "hashes::sha1::tests::rejects_extra_suffix_bytes"
       ],
       "references": [
         "fips-180-4",
@@ -1771,6 +1773,26 @@
           "metric_keys": [
             "sha1_u32_32"
           ]
+        },
+        {
+          "id": "message-80-midstate",
+          "label": "64-byte prefix plus 16-byte suffix from midstate",
+          "parameters": {
+            "prefix_bytes": 64,
+            "suffix_bytes": 16
+          },
+          "includes": "fragment-only: one-block midstate continuation, final padding, and digest restoration; excludes prefix authentication, suffix pushes, digest comparison, and terminal predicate",
+          "script_bytes": 209619,
+          "witness_bytes": null,
+          "witness_bytes_max": null,
+          "max_stack_items": null,
+          "executed_opcodes": null,
+          "validation_weight": null,
+          "setup_script_bytes": null,
+          "per_use_script_bytes": 209619,
+          "metric_keys": [
+            "sha1_u32_80_midstate"
+          ]
         }
       ],
       "limitations": [
diff --git a/knowledge/comparisons/hashes.md b/knowledge/comparisons/hashes.md
index b2e332e..16989c8 100644
--- a/knowledge/comparisons/hashes.md
+++ b/knowledge/comparisons/hashes.md
@@ -7,6 +7,7 @@ Measured fragments exclude input pushes and output comparison.
 | BLAKE3 sparse direct u4 | 32-byte input | 59,529 | differentially-validated | Fixed length at generation time; at most 32 bytes |
 | BLAKE3 limb29 | 64-byte input | 72,293 | differentially-validated | Single 1,024-byte chunk only; includes table memory |
 | SHA-1 u32 | 32-byte input | 209,726 | differentially-validated | Collision-broken compatibility hash |
+| SHA-1 u32 midstate | 64-byte prefix + 16-byte suffix | 209,619 | differentially-validated | Requires an authenticated one-block midstate; collision-broken |
 | RIPEMD-160 u32 | 32-byte input | 244,063 | differentially-validated | 160-bit output |
 | SHA-256 u4 | 32-byte input | 332,942 | differentially-validated | Large research fragment |
 | SHA-256 u32 | 32-byte input | 512,428 | differentially-validated | Larger than local u4 variant |
diff --git a/knowledge/primitives/sha1-u32.md b/knowledge/primitives/sha1-u32.md
index b225b14..257d911 100644
--- a/knowledge/primitives/sha1-u32.md
+++ b/knowledge/primitives/sha1-u32.md
@@ -7,10 +7,13 @@ bytes.
   collision resistance is required.
 - **Evidence:** differentially validated against standard reference digests and
   internal round checks.
-- **Representative result:** a 32-byte hashing fragment is 209,726 bytes.
+- **Representative results:** a 32-byte hashing fragment is 209,726 bytes; a
+  16-byte suffix continuation from a one-block midstate is 209,619 bytes.
 - **Deployment:** operation-heavy research fragment; complete consensus and
   policy feasibility are configuration-dependent and not established here.
-- **Stack contract:** one byte item per input byte; 20 byte items returned.
+- **Stack contract:** one byte item per input byte; 20 byte items returned. The
+  midstate continuation consumes exactly 16 suffix bytes and returns the
+  digest for a 64-byte prefix plus that suffix.
 
 See the [implementation README](../../src/hashes/sha1/README.md) and catalog
 record `hash/sha1-u32`.
diff --git a/src/hashes/sha1/README.md b/src/hashes/sha1/README.md
index 24560b7..7ac866e 100644
--- a/src/hashes/sha1/README.md
+++ b/src/hashes/sha1/README.md
@@ -19,6 +19,7 @@ digest comparison.
 | Configuration | Hashing script |
 | --- | ---: |
 | 32-byte input | <!-- metric:sha1_u32_32 -->209726<!-- /metric:sha1_u32_32 --> bytes |
+| 64-byte prefix + 16-byte suffix from midstate | <!-- metric:sha1_u32_80_midstate -->209619<!-- /metric:sha1_u32_80_midstate --> bytes |
 
 This fragment exceeds the repository optimizer's 32 KiB input cutoff and is
 reported unoptimized.
@@ -58,6 +59,12 @@ value in `0..=255`.
 temporary lookup table and message schedule are removed, and the altstack is
 restored to its starting depth.
 
+`sha1_80bytes_from_midstate(midstate)` consumes exactly 16 suffix bytes and
+continues from the state after a 64-byte prefix. The generated fragment
+includes the final 80-bit length encoding for the resulting 80-byte message;
+the caller is responsible for authenticating the supplied midstate and its
+prefix binding.
+
 ## Operational notes
 
 Padding uses SHA-1's big-endian length encoding with a zero high 32-bit word,
diff --git a/src/hashes/sha1/mod.rs b/src/hashes/sha1/mod.rs
index 6cae303..24b13bb 100644
--- a/src/hashes/sha1/mod.rs
+++ b/src/hashes/sha1/mod.rs
@@ -58,6 +58,39 @@ pub fn sha1(num_bytes: usize) -> Script {
     }
 }
 
+/// Continues SHA-1 from the state after one 64-byte block over a 16-byte
+/// suffix, producing the digest of the resulting 80-byte message.
+pub fn sha1_80bytes_from_midstate(midstate: [u32; 5]) -> Script {
+    let mut state = midstate;
+    state.reverse();
+    script! {
+        { push_reverse_bytes_to_alt(16) }
+        { u8_push_xor_table() }
+        for _ in 0..16 {
+            OP_FROMALTSTACK
+        }
+        0x80
+        { push_to_stack(0, 39) }
+        { u32_push(0) }
+        { u32_push(640) }
+        for i in 1..16 {
+            { u32_roll(i as u32) }
+        }
+        for word in state {
+            { u32_push(word) }
+        }
+        { sha1_transform(16) }
+        { sha1_final() }
+        for _ in 0..5 {
+            { u32_toaltstack() }
+        }
+        { u8_drop_xor_table() }
+        for _ in 0..5 {
+            { u32_fromaltstack() }
+        }
+    }
+}
+
 fn push_reverse_bytes_to_alt(num_bytes: usize) -> Script {
     script! {
         for i in 1..=num_bytes {
@@ -307,6 +340,7 @@ fn majority(words_above_table: usize) -> Script {
 mod tests {
     use super::*;
     use crate::arithmetic::u32::stack::{u32_equal, u32_push};
+    use crate::support::execution::execute_script_with_inputs;
     use bitcoin::hashes::{sha1 as reference_sha1, Hash};
 
     fn push_message(message: &[u8]) -> Script {
@@ -332,6 +366,40 @@ mod tests {
         assert!(result.success, "{result}");
     }
 
+    fn compress(state: &mut [u32; 5], block: &[u8; 64]) {
+        let mut words = [0u32; 80];
+        for (word, bytes) in words.iter_mut().zip(block.chunks_exact(4).take(16)) {
+            *word = u32::from_be_bytes(bytes.try_into().unwrap());
+        }
+        for t in 16..80 {
+            words[t] = (words[t - 3] ^ words[t - 8] ^ words[t - 14] ^ words[t - 16]).rotate_left(1);
+        }
+
+        let [mut a, mut b, mut c, mut d, mut e] = *state;
+        for (t, word) in words.iter().enumerate() {
+            let function = if t < 20 {
+                d ^ (b & (c ^ d))
+            } else if t < 40 || t >= 60 {
+                b ^ c ^ d
+            } else {
+                (b & c) | (d & (b | c))
+            };
+            let constant = round_constant(t);
+            let next = a
+                .rotate_left(5)
+                .wrapping_add(function)
+                .wrapping_add(e)
+                .wrapping_add(constant)
+                .wrapping_add(*word);
+            (a, b, c, d, e) = (next, a, b.rotate_left(30), c, d);
+        }
+        state[0] = state[0].wrapping_add(a);
+        state[1] = state[1].wrapping_add(b);
+        state[2] = state[2].wrapping_add(c);
+        state[3] = state[3].wrapping_add(d);
+        state[4] = state[4].wrapping_add(e);
+    }
+
     #[test]
     fn round_functions_match_reference() {
         let a = 0x0123_4567u32;
@@ -448,6 +516,46 @@ mod tests {
         verify_digest(&[0x24; 130]);
     }
 
+    #[test]
+    fn continues_from_midstate() {
+        let prefix = [0x42u8; 64];
+        let suffix: Vec<u8> = (0..16).collect();
+        let mut midstate = INITIAL_STATE;
+        compress(&mut midstate, &prefix);
+        let mut message = prefix.to_vec();
+        message.extend_from_slice(&suffix);
+        let expected = reference_sha1::Hash::hash(&message).to_byte_array();
+        let result = crate::support::execution::execute_script_without_stack_limit(script! {
+            { push_message(&suffix) }
+            { sha1_80bytes_from_midstate(midstate) }
+            for byte in expected {
+                { byte }
+                OP_EQUALVERIFY
+            }
+            OP_TRUE
+        });
+
+        assert!(result.success, "{result}");
+    }
+
+    #[test]
+    fn rejects_extra_suffix_bytes() {
+        let result = execute_script_with_inputs(
+            script! {
+                { sha1_80bytes_from_midstate(INITIAL_STATE) }
+                for _ in 0..20 {
+                    OP_DROP
+                }
+                OP_DEPTH
+                OP_0
+                OP_EQUAL
+            },
+            vec![vec![0x42]; 17],
+        );
+
+        assert!(!result.success);
+    }
+
     #[test]
     fn rejects_unsupported_message_length() {
         let panic = std::panic::catch_unwind(|| sha1(512));
diff --git a/tests/primitive_metrics.rs b/tests/primitive_metrics.rs
index 581dbce..62cc81a 100644
--- a/tests/primitive_metrics.rs
+++ b/tests/primitive_metrics.rs
@@ -3559,6 +3559,11 @@ fn metrics() -> Vec<Metric> {
             key: "sha1_u32_32",
             value: script_len(sha1::sha1(32)),
         },
+        Metric {
+            readme: "src/hashes/sha1/README.md",
+            key: "sha1_u32_80_midstate",
+            value: script_len(sha1::sha1_80bytes_from_midstate([0; 5])),
+        },
         Metric {
             readme: "src/hashes/sha256/README.md",
             key: "sha2_u32_32",