diff --git a/shared/stargate/build.gradle.kts b/shared/stargate/build.gradle.kts
index 41ade1a..becc940 100644
--- a/shared/stargate/build.gradle.kts
+++ b/shared/stargate/build.gradle.kts
@@ -20,11 +20,16 @@ kotlin {
 
     sourceSets {
         commonMain.dependencies {
+            implementation(projects.shared.crypto)
+
+            implementation(libs.cryptography.bigint)
             implementation(libs.ktor.client.cio)
             implementation(libs.ktor.client.core)
             implementation(libs.ktor.client.websockets)
+        }
 
-            implementation(libs.cryptography.bigint)
+        commonTest.dependencies {
+            implementation(libs.kotlin.test)
         }
     }
 }
diff --git a/shared/stargate/src/commonMain/kotlin/net/agorise/shared/stargate/mnemonics/Mnemonics.kt b/shared/stargate/src/commonMain/kotlin/net/agorise/shared/stargate/mnemonics/Mnemonics.kt
index be2bd17..69be353 100644
--- a/shared/stargate/src/commonMain/kotlin/net/agorise/shared/stargate/mnemonics/Mnemonics.kt
+++ b/shared/stargate/src/commonMain/kotlin/net/agorise/shared/stargate/mnemonics/Mnemonics.kt
@@ -2,6 +2,8 @@ package net.agorise.shared.stargate.mnemonics
 
 import dev.whyoleg.cryptography.bigint.BigInt
 import dev.whyoleg.cryptography.bigint.toBigInt
+import io.ktor.utils.io.core.toByteArray
+import net.agorise.shared.crypto.crc.Crc32
 
 /**
  * Provides functionality to create and validate mnemonics.
@@ -115,33 +117,31 @@ class Mnemonics {
 //
 //        return calculatedChecksumWord == checksumWord
 //    }
-//
-//    /**
-//     * Calculates a checksum (using CRC algorithm) on first 24 words.
-//     */
-//    @OptIn(ExperimentalUnsignedTypes::class)
-//    private fun calculateChecksumIndex(words: Array<String>, prefixLen: Int): Result<UInt> {
-//        val trimmedRunes = mutableListOf<Char>()
-//
-//        if (words.size != SEED_LENGTH) {
-//            return Result.failure(Exception("Words not equal to seed length"))
-//        }
-//
-//        for (word in words) {
-//            val trimmedWord = if (word.length > prefixLen) {
-//                word.substring(0, prefixLen)
-//            } else {
-//                word
-//            }
-//            trimmedRunes.addAll(trimmedWord.toCharArray().toList())
-//        }
-//
-//        val checksum = CRC32().apply {
-//            update(trimmedRunes.toCharArray().concatToString().toByteArray(Charsets.UTF_8))
-//        }.value
-//
-//        return Result.success(checksum % SEED_LENGTH.toUInt())
-//    }
+
+    /**
+     * Calculates a checksum using CRC-32 algorithm on the seed (SEED_LENGTH words) as follows:
+     * - Takes prefixLen chars (not bytes) from each word and concatenates them
+     * - Calculates the CRC-32 checksum on the concatenated bytes
+     * - Returns the mod of checksum by SEED_LENGTH, to get the checksum word
+     */
+    internal fun calculateChecksumIndex(words: List<String>, prefixLen: Int): Result<UInt> {
+        if (words.size != SEED_LENGTH) {
+            return Result.failure(Exception("Words not equal to seed length"))
+        }
+
+        val trimmedWords = mutableListOf<Char>()
+
+        words.forEach { word ->
+            trimmedWords.addAll(word.take(prefixLen).toList())
+        }
+
+        // TODO Use DI to get an instance of CRC32
+        val crc32 = Crc32()
+
+        val checksum = crc32.crc32(trimmedWords.joinToString("").toByteArray())
+
+        return Result.success(checksum % SEED_LENGTH.toUInt())
+    }
 
     companion object {
         // Each language should have exactly this number of words
diff --git a/shared/stargate/src/commonTest/kotlin/net/agorise/shared/stargate/mnemonics/MnemonicsTest.kt b/shared/stargate/src/commonTest/kotlin/net/agorise/shared/stargate/mnemonics/MnemonicsTest.kt
new file mode 100644
index 0000000..e3a9188
--- /dev/null
+++ b/shared/stargate/src/commonTest/kotlin/net/agorise/shared/stargate/mnemonics/MnemonicsTest.kt
@@ -0,0 +1,40 @@
+package net.agorise.shared.stargate.mnemonics
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertTrue
+
+class MnemonicsTest {
+    private val mnemonics = Mnemonics()
+
+    @Test
+    fun `given an invalid seed - when calculateChecksumIndex is called - then result is failure`() {
+        val invalidSeed = "invalid seed".split(" ")
+
+        val result = mnemonics.calculateChecksumIndex(invalidSeed, 3)
+
+        assertTrue(result.isFailure)
+    }
+
+    @Test
+    fun `given a valid english seed - when calculateChecksumIndex is called - then result is correct`() {
+        val expectedChecksumIndex = 22u
+        val seed = "sequence atlas unveil summon pebbles tuesday beer rudely snake rockets different fuselage woven tagged bested dented vegan hover rapid fawns obvious muppet randomly seasons randomly".split(" ").dropLast(1)
+
+        val result = mnemonics.calculateChecksumIndex(seed, 3)
+        val actualChecksumIndex = result.getOrThrow()
+
+        assertEquals(expectedChecksumIndex, actualChecksumIndex)
+    }
+
+    @Test
+    fun `given a valid spanish seed - when calculateChecksumIndex is called - then result is correct`() {
+        val expectedChecksumIndex = 7u
+        val seed = "perfil lujo faja puma favor pedir detalle doble carbón neón paella cuarto ánimo cuento conga correr dental moneda león donar entero logro realidad acceso doble".split(" ").dropLast(1)
+
+        val result = mnemonics.calculateChecksumIndex(seed, 4)
+        val actualChecksumIndex = result.getOrThrow()
+
+        assertEquals(expectedChecksumIndex, actualChecksumIndex)
+    }
+}